Methionine is currently produced as a DL-methionine racemic mixture by a well-established chemical process that involves toxic, dangerous, flammable, unstable, and noxious materials or intermediates. The starting materials for the chemical production of methionine are acrolein, methylmercaptan, and hydrogen cyanide. The chemical synthesis of methionine involves the reaction of methylmercaptan and acrolein producing the intermediate 3-methylmercaptopropionaldehyde (MMP). Further processing involves reacting MMP with hydrogen cyanide to form 5-(2-methylthioethyl) hydantoin, which is then hydrolysed using caustics such as NaOH together with Na2CO3, NH3, and CO2. Subsequently, sodium DL-methionine is neutralized with sulfuric acid and Na2CO3 to yield DL-methionine, Na2SO4, and CO2. This process yields a large excess of unused compounds in comparison to the amount of methionine produced that poses an economic and ecological challenge.
Fermentative processes for methionine production are typically based on cultivating microorganisms with nutrients including carbohydrate sources, e.g., sugars, such as glucose, fructose, or sucrose, nitrogen sources, e.g., ammonia, and sulfur sources e.g., sulfate or thiosulfate, together with other necessary media components. This process yields L-methionine and biomass as a byproduct with no toxic dangerous, flammable, unstable, and/or noxious starting materials.
However, in order for an organism (e.g., a microorganism) to produce methionine from sulfate as a sulfur source, the sulfur atom must be first reduced to sulfide. This process is energy intensive, so that feeding the microorganism a sulfur source that is more reduced than sulfate would improve the process. One such reduced sulfur source is thiosulfate, in which one of the two sulfur atoms is already reduced. Another source of reduced sulfur is methane thiol, which contains a fully reduced sulfur atom.
The use of methane thiol for the production of methionine offers two advantages. First, as mentioned above, the sulfur atom is already reduced. Second, a methyl group is supplied, which could potentially bypass the need for two of the enzymes that are normally required for methionine biosynthesis, methyltetrahydrofolate reductase (MetF) and methionine synthase (MetE and/or MetH). There are literature reports that disclose that some microorganisms, for example Saccharomyces cerevisiae, can enzymatically incorporate methane thiol directly into methionine by reacting it with O-acetyl homoserine (Yamagata, S. 1971. J. Biochem. (Tokyo) 70:1035). Methods for the use of methane thiol in the production of methionine are also disclosed in WO 93/17112 and WO 2004/076659.
However, the use of methane thiol for the production of methionine also has disadvantages. It is a toxic, explosive gas that readily oxidizes in air, and it is noxious. The chemical process for producing methionine also uses methane thiol as one of the substrates, so engineers have learned to handle the compound on an industrial scale. Nonetheless, improved processes for the production of methionine that do not use methane thiol would be of great benefit.